Rather annoyingly, the compositor support table on this page seems to be showing only the latest version of each compositor (plus or minus a month or two, e.g. it's behind on KWin). I assume support for the protocol predates these versions for the most part? Do you know when the first versions of KDE and Gnome to support the protocol were released? Asking because some folks in this thread have claimed that a large majority of shipped Wayland systems don't support it, and it would be interesting to know if that's not the case (e.g. if Debian stable had support in Qt and GTK applications).

We first shipped support for wp-fractional-scale-v1 in Plasma 5.27 in early 2023, support for it in our own software vastly improved with Plasma 6 (and Qt 6) however.

Fractional scaling is the problem, not the solution! It replaces rendering directly at the monitor’s DPI, which is strictly better, and used to be well-supported under Linux.